Multirange resistance thermometer



Nov. 3, 1953 G. w. scHRoEDER 2,657,580

MULTIRANGE RESISTANCE THERMOMETER Filed Nov. 2, 1951 A s sheets-snaai 135% 32 so ea /3 J S .g JAVA A'qw Q m j Z :l 41594 OFF 96 c JNVENTOR. FAT597 J 'S4 ,92, T Georg W Schroeder CANDY 5W BY' MEAT 9a l f 9971 ,05lswdl/yz! M, y/d/Yd Nov. 3, 1953 G. w. scHRoEDER MULTIRANGE RESISTANCETHERMOMETER 5 Sheets-Sheet 2 Filed Nov. 2. 1951 4 m G. e W

, E I w George W Schroeder Alfys. n

Nov. 3, 1953 G. w. scHRoEDER 2,657,580

MULTIRANGE RESISTANCE THERMOMETER Filed Nov. 2, 1951 3 Sheets-Sheet 3RARE BEEF VEAL WELL DONE BEEF 73 MEDIUM BEE PORK HAM LAMB POULTRY -7--7- 1 2 3 4 6 7 8 soFT FIRM HARD soFT HARD BALL BALL BALL cRAcK cRAcK l1 l I 1 j l l DONUTS PoTAToEs E L FRITTERs HBH CROQUETTS oFF i 59 52 1 FI G 6 INVENToR. 55 57 58 George W Schroeder 50l BY Patented Nov. 3, 1953vMULTIRANGE RESISTANCE THERMOMETER George W- Schroeder, Louisville,.Ky., @Signor to GeneralwElectrie Company, -a corporation of New YorkApplication November 2, 1951, Serial No.- 254,558

7 Claims, (Cl. 73H36@ freached a predetermined ktemperature indicatingthat the cooking of the-roast .has proceeded a corresponding 'degreefrom Athe raw state into a rare, "medium or Well-donecondition.Similarly incarrying-fout 'a fat-'frying or a candymaking cookingoperation in a vessel that is heated on the hot platecarried by thecooking top ofthe electric range, i-tfis' conventional practice toinsert a mechanical y'thern-iometer into the `vessel so that vtheYtemperature of the -fat or ofthe candy syrup Amay be' determined andmaintained ,during the cooking operation.

While these `arrangements are vreasonably satisfactory, considerablecare must be exercised b y the cook in'carrying out the cookingoperations in order to obtain good 'cooking results. curately to readvthe thermometers vinvolved in these operations due to the presence ofsteam, vapor, etc., issuing from the food being cooked. Moreover, incarryingou-t the cooking of a roast, the cook must repeatedly open'theoven door and slide the roast from-the oven chamber in order to rendervisible the thermometervthat has' been inserted 'therein-to.

It is a generali object of the present invention -to provide an electrictemperature indicating system including a 'temperature responsive'electrical element, an electric-meter providedvvith a 'number ofscales, a `circuit network provided withk a number of positionsrespectively matching the electrical element to the diierent scales ofthe meter, anda single manually controlled member for selecting any oneof thesca'les of lthe meter and vfor simultaneously -settingthe circuitnetwork into 'the `corresponding one of its circuit positions. Anotherobject of the invention is to provide an electric vtemperatureindicating-systemthat includes an Edison three-Wirealternating currentsource and an improved arrangementfor operating therefrom a voltageamplifier and -a Also it is quite diicult for the vcool; ac-

2 ofthe character noted that incorporates -an an rangement for`preventing the outputs of the amplifiers Afrom increasingAproportimually with an increase in voltage above normal between the.outside conductors of the three-.Wire Edison source.

Further features o the invention pertain 112,0 lthe particulararrangement of the y.elements 4of the electrical .temperature indicatingsystem, whereby the `.abov.e=.; ut1ined and additional operatingfeatures thereof are attained.

.The invention, vboth as lto its :organization and method of operation,together with further ob.- jects and advantages thereof, `Will vbest beunder.- stood by reference to the following specification taken inconnection With the accompanying drawings, in which 'Figure 1 is a fronteleva.,- tional view of an electric range incorporating an yelectricaltemperature indicating system and embodying the'present invention; Fig.2 vis an .eiectrical diagram of the circuit arrangement `or the elementsof` the temperature indicating Asystem incorporated in the electricrange; Fig. 3 is an enlarged fragmentary front elevational view, partly'broken away, .oi a portion .of the electric temperature indicatingmeter and the 'circuit controller incorporated in that indicat-:ing1system; Fig. .4 is an enlarged fragmentary sectional View,V takenin the .direction V.of the arrows along vthe oiset line ll-f-li in Fig.B, of Ithe lportion of the electric temperattute indicating fmeteraandthe circuit controller; Fig. .5 -is a fur.- ther enlarged fragmentaryfront .eleva-tional viewsof the movable lscale .carrying member that vis.incorporated in .the electric temperature indicating meter shown inFigs. .3 and 4; and Fig.

:.6 is anenlarged fragmentary view, partly in section, of .thetemperaturesensing probe and A c on.- nected flexible armored cable and`circuit conhector arrangement that is incorporated in .the electrical.temperature indicating system, .as shown in Figs. 1 and 2.

.Referring now to iFig. 1 -of :the drawings, vthe .electric range .Iiithereillustrated and .incorporating .theelectrical temperatureindicating system .embodying the features .of the present invention.comprises an vnpstanding rnetalbody il that is provided lwith asubstantially horizontally dis- -posed metal cooking 4top i2 which:terminates adjacent to the rear edge thereof in a rlaterall-y extendingnpstanding hollo-vv metal backsplash I3.

4The' body I [houses in the upper right-hand .por-

tion thereof a .broiling yand baking .oven .cavity l.or .chamber YJil,which .oven .chamber i4 is y.provided witha door J5 that is hinged.adjacent to. the

lower edge thereof and movable between open and closed positions, thedoor I5 being illustrated in its open position. Also the body I I mayhouse in the upper left-hand portion thereof a longitudinally slidabledrawer that is provided with a front panel IB substantially matching thedoor I5 in its closed position. Finally the body II may house in thelower right-hand portion thereof and in the lower left-hand portionthereof two longitudinally slidable drawers that are respectivelyprovided with front panels I1 and I8 respectively disposed below thedoor I5 and below the panel I6.

Arranged in the lower portion of the oven cavity I4 is a longitudinallyslidable and removable supporting rack I9 carrying an electric heatingunit that is preferably of the sheathed resistance conductor type; andarranged in the upper portion of the oven cavity I4 is a longitudinallyslidable and removable rack 2I carrying an electric heating unit 22 thatis preferably of the sheathed resistance conductor type. The heatingunits 20 and 22 are provided with electrical terminal structures, notshown, that are respectively received in electrical receptacles, notshown, supported by the rear wall of the metal liner or shell |411 thatdefines the oven cavity I4 in a conventional manner. Also the left-handwall of the liner I4 carries adjacent to the door I5 and within the ovenchamber I4 an electrical connector or receptacle 23 that comprises aportion of the electrical temperature indicating system incorporated inthe electric range I0, as explained more fully hereinafter.

The cooking top I2 carries on the left-hand side thereof, four electrichot plates 24, 25, and 21, arranged in a rectangular pattern in aconventional manner, the hot plates 24 and 25 being respectivelyarranged in left front and right front positions, and the hot plates 25and 21 being respectively arranged in left rear and right rearpositions. Preferably the right-hand side of the cooking top I2 isuninterrupted to provide a suitable work supporting surface.

lThe backsplash I3 supports on the right-hand side thereof an ovenswitch 28 and four hot plate switches 29, 30, 3I and 32 individual tothe respective hot plates 24, 25, 26 and 21 for the usual circuitcontrol functions, as well as a conventional oven temperature regulatingthermostatic switch 33 and receptacle outlet 34. Also the backsplash I3supports on the left-hand side thereof the usual electric oven timerswitch 35 of the clock-controlled type for the purpose of controllingoperation of the heating units 20 and 22 in the oven chamber I4 upon atime basis. Finally the central portion of the backsplash I3 `supportsan electric temperature indicating meter 3B, a cooperating circuitcontroller 31, and an associated electrical connector or receptacle 38,the elements 36, 31 and 38 forming a part of the electrical temperatureindicating system incorporated in the electric range I0, as explainedmore fully hereinafter. Further the electric range I0 is provided with athree-wire Edison supply circuit that is preferably of 230 volts, 60cycles, single phase A. C., the supply circuit including the usualgrounded neutral conductor 39 and two ungrounded outside conductors 40and 4I, as indicated in Fig. 2.

Further the temperature indicating system comprises, as illustrated inFigs. l, 2 and 6, an elongated substantially cylindrical metaltemperature sensing probe 42 that is connected to an electricalconnector or plug 43 by a suitable length of flexible metal armoredcable 44, the plug 43 being selectively connectible to the receptacles23 and 38 for the purpose of selectively associating the temperaturesensing probe 42 respectively with the oven chamber I4 and with thebacksplash I3. More particularly when the plug 43 is inserted into thecooperating receptacle 23 the temperature sensing probe 42 isoperatively connected into the electrical temperature indicating system,and at this time the flexible armored cable 44 permits the readyinsertion of the probe 42 into a roast of meat indicated at 45 supportedin a suitable roasting pan indicated at 45 and supported upon the lowerrack I5 arranged in the oven chamber I4. At this time the temperature ofthe probe 42 is readable upon the electrical temperature indicatingmeter 35 after appropriate setting of the cooperating circuit controller31 in a manner more fully explained hereinafter, whereby the ovencooking operation may be readily observed by the cook in an obviousmanner. On the other hand, when the plug 43 is inserted into thecooperating receptacle 33, the temperature sensing probe 42 isoperatively connected into the electrical temperature indicating system,and at this time the iiexible armored cable 44 permits ready insertionof the probe 42 into food being cooked in a saucepan or other vessel 41supported by one of the hot plates 24, etc., carried by the cooking topI2. At this time the temperature of the probe 42 is readable upon theelectrical temperature indicating meter 35 after appropriate` setting ofthe cooperating circuit controller 31 in a manner more fully explainedhereinafter, whereby the hot plate cooking operation may be readilyobserved by the cook in an obvious manner.

`Suitable constructional details of the probe 42, the armored cable 44,the plug 43, and the receptacle 23 are shown in Fig. 6. Morespecifically the probe 42 comprises an elongated cylindrical casing 48formed of Invar, a nickel-steel alloy consisting essentially ofapproximately 36-40% nickel and 60-64% iron, or other metal having a lowthermal conductivity, and a pointed tip or arbor 49 formed of aluminumor other metal having a high thermal conductivity, the inner end of thearbor 49 being suitably secured in the outer end of the casing 43. Theinner end of the casing 48 is closed by a ferrule 49 that may be formedof Invar and suitably secured to the metal armor of the flexible cable44. -A tube 50 is suitably supported in the outer end of the casing 48between the arbor 43 and an adjacent washer 5I, the tube 50 and thewasher 5I being formed of lava or other suitable electrical insulatingmaterial. A temperature sensing resistor 52 is wound upon the tube 50and housed within the outer end of the casing 48, the resistor 52 beingarranged in good heat-conducting relation with the arbor 49. Thetemperature sensing resistor 52 has a high positive coefcient ofresistance and preferably consists of about 5-ft. of No. 40 Hytemcooxide coated wire, a nickelsteel alloy that consists essentially ofapproximately '10% nickel and 30% iron. The resistor 52 has a verynearly linear temperature versus resistance characteristic between '10and 1300 F.; the electrical resistance thereof at 1300 F. beingsubstantially five times that at '10 F. The resistor 52 has a resistanceof 50 ohms and the opposite terminals thereof are electrically connectedto a pair of electrically insulated conductors 53 and 54 extending intothe adjacent end of the armored cable 44.

`lThe plug 43 consists essentially of a 'substan- 'tially cylindricalmetal outer shell V55 that is suitably joined to the metal armor of theadjacent end of the Acable 44 and houses 'a block of electricalinsulating material 55 that supports a pair of electrical plug prongs 56connected to the adjacent ends ofthe pair of'conductors 53 Vand 54disposed in the armored cable 44. Similarly 'the receptacle 23 consistsessentially of a "substantially cylindrical metal outer shell 51provided-With a supporting iiange 58 and housing a block of electricalinsulating material 59 that supports a pair of electrical jack prongs60. It will be 'understoodthat the supporting flange 58 issuitablysecured tothe adjacent wall of the 'metal liner |4a ofthe ovencavity I4 grounding lthe shell 51 to the frame of the electric range I0.When the plug 43 is inserted into the receptacle 23, the Vshells '55 and51 make good electrical con- "t'act with each other grounding out thearmor of 'the cable 44 and the exterior of the probe 42 and completingan electric circuit between the exterior and 'the resistor 52 via theprongs l56-and 63 and the conductor pair 53--54. This arrangementpositivelyr prevents any potentials impressed from the exterior upon theresistor 52 from'appearing upon the exterior of the plug 43, the armoredcable 44 or the probe 42, so that Vthere is no danger of electricalshocks 'to the cook incident to vhandling the probeA 42 and 'placementand removal of the plug 43 with respect to the receptacle 23.

In view of the foregoing description of the 'construction andarrangement of the receptacle 23 carried by the 'wall of the metal ovenliner 14a, it will be understood that the receptacle 38 'carried bythemetal backsplash I3 is identical, whereby the plug 43 may be readilyinserted into the receptacle 38 so as to connect the resistor 52 into anexterior circuit and to ground out the armored cable 44 and the probe 42upon the grounded receptacle 38 via vthe metal backsplash 'I3 and themetal frame of the range I0.

`Suitable constructional details of the electrical'temperatureindicati'ng meter 36 and the circuit controller 31 are shownirl-Figs. 3, 4 and 5. More v`specAiically the meter 36 comprises an`opaque 4face plate 6| `having a window '62 formed ,therethrough, whichface plate 6| preferably com- -`prises-a portion of the metal sheetconstituting the front ofthe backsplash |13; also the meter 36 includes'an 'electroresponsive D. C. milliam- .meter 63 having a'range 0-24milliamperesand `A#disposedbehind"the face plate l I. Themilliami'rie'ter 63 is provided with a rotatably mounted shaft 64projecting through an associated hole formed 'inthe face plate .6I andcarrying an indicatorGS, the outer vend of which cooperateswith "thewindow |52 formed in the face plate 6 I. Preferably 'the face plate 6|and the indicator 55 dis- 'fposedadjacent tothe face 'thereof arecovered by a rectangular pane of'gla'ss 66, which is suitably 'supportedin a vrectangular frame 61 .carried by 4'slide 68 comprises `fourv'vertically spaced-apart indicator-.scales 10, 1|, 12 vand 13 on thefront V''the'reoftl'lat selectively cooperate with' the window informedthe faceplate TGI. "More specically the scale FID comprises an olf scaleyand carries the simple 'legend ofE; the scale 1| `comprises a fat scaleand `carries the appropriate legends nshj etc., indicatedcorrespondingto fatfrying cooking operations; the scale 12 comprises a candy scaleand carries the appropriate legends hard ball, etc., indicatedcorresponding to candy cooking operations; and the scale 13 comprises ameat scale and carries the appropriate legends veal, etc., indicatedcorresponding to meat cooking operations.

The circuit controller 31 consists essentially of -a suitablefourepo'stition rotary snap switch 14 disposed rearwardly of the faceplace 6| andprovided with a rotatably mounted shaft 15 extending throughvaligned holes respectively formed vin the face plate 6| and in theglass pane fthe outer'end of the shaft 15 carrying a manuallyoperable-handle 16. The outer end of the manually operable handle 16constitutes an index pointer that lcooperates with circumferentiallyspaced-apart indicia om Afat, candy and meat provided on the adjacentsurface of the glass pane 66. In View of the foregoing, itwill beunderstood vthat the handle 16 is lselectively rotatable into the fourpositions noted, whereby the pointer provided on the outer end thereofcooperates with the respective legends voiff etc.. noted, and the snapvswitch 14 vis operated into its corresponding and respective electricaloil. etc., positions. Alsoa gear 11is rigidly fixed to the shaft 15 byan arrangement including a set screw 10, the gear l1 being disposedbehind the face plate 6| and cooperating with a rack 19 formed on theright-hand side of the indicator slide 53 as an integral part thereof.This arrangement provides a positive drive between the shaft 15 of thecircuit controller 31 and the irldicator slide 88, whereby the scales10, etc., provided on the indicator slide 68 are respectively framed inthe window 62 provided in the face plate 6| when the handle 16 isrespectivelyoperated into its corresponding positions. Specifically asillustrated in Figs. 3 and 4, when the handle 16 occupies its olfposition, the cooperation between the gear 11 and the rack 19 bringstheoi scale 10 provided on the indicator slide 68 into registry with thewindow E2. As the handle 'T6 is vmoved into its successive positions inthe clockwise direction, as viewed in Fig. 3, the gear 11 cooperatingwith the rack 1S lowers the indicator slide 68 successively framing thescales 1|, v12 and v13 in the window 62. 'In passing, it is noted thatthe scale spread of each of the scales 1|, 12 and 13 carried by theindicator slide 68 comprises approximately F.; the fat cookingoperations normally extending from approximately '320 to 420 F.; thecandy cooking operations normally extending from approximately 220 to320 F.;

and the meat cooking operations normally extending from approximately to220 F.

Referring now to Fig. 2,'the circuit network that operativelyinterconnects the three-wire Edison source, the electrical temperatureindicating meter 3B, the circuit controller 31, the temperature sensingresistor 52 housed in the probe 42, the plug 43, and one of thereceptacles 23 or 3-8 also compri-se two electron vdischarge devicesl|30 and 8|. The electron discharge device or'tube 80 lconstitutes atriode that is preferably of the IZA'VG type and is connected as aVoltage amplifier; and the electron discharge device or tube 8|constitutes a pentode that is preferably of lthe 50C5 type Aand isconnected as a 'power' amplifier. More particularly, the tube 80comprises an anode or plate 582, a cathode 83 of 'the thorium,.etc.,

7 oxide coated type provided with a heater 811, and a control grid 85;and the tube 8l comprises an anode or plate S6, a cathode 8l of thethorium, etc., oxide coated type provided with a heater S8. a controlgrid 89, a screen grid 99 and a suppressor grid 9I.

In the arrangement of the meter 35, the position of the indicator 65upon the shaft 34 is such that the response of the milliamrneter 63 isdepressed by 4 milliamperes so that the indicator 65 reads over the fullrange of the selected scales TI, etc., in response to current traversingthe milliammeter 63 over the range 4-24 milliarnperes, since the gridvoltage versus plate current characteristic of the tube 8l issubstantially linear over the plate current range Li-2l milliamperes,whereby the scales 'I I, etc., are substantially linear when calibratedin degrees F., as explained more fully hereinafter.

In the circuit network the snap switch 'I4 of the circuit controller 3?comprises 3 rotary blades 92, 93 and 913 that are commonly mounted uponthe rotatable shaft 'I5 thereof, the two outside conductors de and ll ofthe Edison source being respectively connected to the two blades 92 and93. In Fig. 2, the ganging of the blades 92, 93 and 94 of the snapswitch 'l and of the control of the scales of the temperature indicatingmeter 36 is indicated by the broken lines "a5 representing the rotatablymounted shaft 'I5 shown in Figs. 3 and 4. Each of the blades 92, S3 and94 is provided with four contacts in the individually associated contactbanks thereof and respectively corresponding to the four positions off,fat, candy and meat of the circuit controller 3l. The fat, candy andmeat contacts associated with the blade 92 commonly terminate aconductor 95; the fat, candy and meat contacts associated with the blade93 commonly terminate a conductor 95; the fat and candy contactsassociated with the blade 9d respectively terminate two conductors el'and 98; and the blade 94 terminates a conductor 99.

The plate 82 of the tube 39 is connected to one terminal of a resistorIQ@ of approximately 4.7 megohms and 1/2 watt, the other terminal of theresistor lili) being connecte-d to the neutral conductor 39; the controlgrid of the tube 89 is directly connected to the conductor 96; and thecathode S3 of the tube t@ is directly connected to the conductor 9?. Iworesistors iti and E92 are connected in series circuit relation betweenthe neutral conductor 39 and the conductor 91, the resistor lili beingadjustable, the resistor I9! being of OGG ohms and 2 watts and theresistor It being of G@ ohms and 2 watts. rThe plate 36 of the tube Siis directly connected to one terminal of the milliarnrneter t3, theother terminal of the milliammeter 63 being directly connected to theconductor 95; the cathode Si of the tube I is directly connected to theneutral conductor 39; the controi grid S9 of the tube ii is directlyconnected to the junction between the one terminal of the resistor ISUand the plate 82 of the tube 3G; the screen grid 9e of the tube BI isdirectly connected to the conductor 9%; and the suppressor grid SI ofthe tube 8i is directly connected to the cathode 8l thereof.

The heaters SII and 3S of the respective tubes 8G and 3i are connectedin series circuit relation between the neutral conductor 3i? and theconductor S8 via a ballast resistor Fsc in the form of a watt lamphaving a high positive temperature coeicient of resistance, for apurpose more fully explained hereinafter. Also a resistor IDA of l5 ohmsand l/2 watt is connected between the conductors 91 and 98; and aresistor |05 of 16 ohms and 1/2 watt is connected between the conductors98 and 99. The conductors 96 and 99 are connected in multiple to thepairs of jack prongs arranged in the receptacles 23 and 3a so that thetemperature sensing resistor 52 is connected via the conductors 53 and54 across the conductors 9G and 99 when the plug i3 is inserted either'into the receptacle 23 or into the receptacle 38. The tubes 96 and SIand the other electrical elements '53, '14, I I, etc., may be suitablymounted within the hollow backsplash I 3.

Considering now the operation of the circuit network of Fig. 2, andassuming that the cook desires to carry out a meat roasting operation inthe oven chamber i4, the cook prepares the roast of meat 45, places itin the vessel 4B, inserts the probe 42 into the roast of meat 45, placesthe vessel GS upon the supporting rack I9 in the oven chamber I4, andinserts the plug 53 into the associated receptacle 23. The oven door I5is then closed and the handle Ity of the circuit 'controller 3l isrotated from its off position into its meat position effecting settingof the switch lades 92, 93 and 94 into their corresponding meatpositions and eiecting setting of the indicator slide 3S so that themeat scale 'I3 is rendered visible in the window 62 of the face plate 6I. The cook then sets the thermostatic oven temperature regulator to thedesired temperature that she wishes to hold in the oven chamber I4.Finally the cool; operates the oven switch 28 in order to eiiectenergization of the electric heating units 20 and 22 bringing aboutheating of the oven chamber lli in the usual manner. At this time theoutside conductors and 4I of the Edison source are respectivelyconnected to the conductors 95 and Q6 since the blades 92 and 93 occupytheir meat positions; and the series circuit is completed between theneutral conductor 39 and the outside conductor I of the Edison sourcevia the resistors Il and 92, the conductor 91, the resistor IM, theconductor 98, the resistor HB5, the conductor 39, the conductor 56, thetemperature sensing resistor 52 and the conductors 53 and 95. Since thecathode 83 of the tube 90 is connected to the conductor 91 andconsequently to an intermediate point in the above-traced circuit, anegative bias is produced between the control grid 85 and the cathode 33of the tube 8G when the voltage applied between the neutral conductor 39and the outside conductor 4I is positive at the neutral conductor 39,whereby during positive loops of the Edison source the tube 8B isrendered conductive so that current flows in the platecathode circuitthereof through the resistor IBI). Also at this time, a negative bias isproduced between the control grid 39 and the cathode 8l of the tube SIby virtue of the Voltage drop across the resistor Ili, whereby duringpositive loops of the Edison source, the tube SI is rendered conductiveso that current ows in the plate-cathode circuit thereof through themilliammeter 63..

As the temperature of the roast of meat 45 rises in the heated ovenchamber I4, the temperature of the interior thereof gradually risesbringing about an increase in the resistance of the temperature sensingresistor 52 housed in the probe d2. As the resistance of the temperaturesensing resistor 52 is thus increased with the temperature thereof, thenegative bias applied between the control grid 85 and the cathode 83 ofthe tube is correspondingly increased, whereby the plate-cathode currentof the tube 80 is correspondingly reduced. thereductionlin theA:platefcathode current of the tube 80 beingsubstam tially linear withthe .increase in the negative bias applied between the-control grid 85andthe cathode 83 thereof, which .increase .in negative bias issubstantially linear withthe increase in resistance of thetem-peraturesensing resistor 52 which increase in resistance of the temperaturesensing resistor 5.2 is substantially linear Awith `the increase vintemperature thereof. As the platecathode current of the tube 80 is thusreduced, the voltagedropacross Ithe resistor is correspondingly reducedin order to bring about a corresponding reduction in the negative biasapplied between the .control grid 89 and the cathode 81 of the tube 8|,whereby the plate-cathode current of the tube 3| traversing themilliarmneter 63 lis correspondingly increased. The decrease in thenegative bias applied between the control grid 89 and the cathode 81 orthe tube 8| .is substantially linear with the decrease in the voltage-drop across the resistor |90; and the increase in the plate-cathodecurrent of the tube 8| is substantially linear with the decrease in the.negative bias applied between the control grid 89 and the'cathode 81thereof. Accordingly the current traversing the milliammeter 53 issubstantially linearly proportional to the temperature of thetemperature sensing resistor 52; whereby the meter 36indicates directlythe temperature of the temper-ature sensing resistor 52.

The presently employed meat scale 13 is matched to the temperaturesensing resistor -52 by virtue of the position of the blade 94 engagingthe meat contact in the associated contact bank,

whereby the resistors |04 and |05 are included in the'above-tracedcircuit establishing` the negative bias betweenthe controll grid 85 andthe cathode 83 of the tube 80. Accordingly the tube 80 functions as avoltage amplifier andthe tube BI functions as Ya power amplier so Vthatthe milliammeter 63 is supplied a current in the range 4-24 milliampereswhen the temperature of the sensing resistor 52 is in the range1209-220`F. While the meat scale 13 is calibrated in terms of thetemperature of the temperature sensing resistor 52, as explained above,the actual values of temperature in degrees F. are omitted fromv thescale 13 in order to avoid confusion on the part of the cook;and'ins-tead the legends rare beef, etc., are substituted therefor.However, it will be understood that the midpoint of the meat scale 13between the legends "veal and well-done beef corresponds to the midpointtemperature of approximately 170 F. in the range In view of theforegoing, it will be understood thatas the temperaturel of thelinterior of the roast of meat'45 rises, the reading of the'meter 36increases, the indicator 65 moving across the meat scale 13 framed inthe window 62 of the face plate 6|, whereby the roast of meat 45 iscompletelycooked lwhen the indicator 65 coincides with the legend on themeatscale 13 that matchesi'boththe characterof the meat of the roast andthev desired* cooking condition thereof.

For example, when the indicator 55 rides'over thev legend veal on themeat scale 13, a veal roastfis completely cooked; whereby at this timethe cook may return` the oven switch 28 back into its off position,sincev the cooking operation hasY been completed.. It will be understoodthat the, 'other legends lamb, etc., carried by the meat scale 13correspond'to therequired. interior` cooking; temperatures of thecorresponding cuts l il fof meat, whereby it is only necessary for thecook to permit the cooking operation to proceed until the indicator '65coincides with the required legend carried by themeat scale 13 asexplained above.

In view of the foregoing, it will be understood that in a similarmanner,the electr-ical temperature indicating system is operative to indicatethe temperature of a cooking operation that is carried out inthe vessel41 supported upon one of the hot plates-24, etc.r Inthis example, theprobe 42 is inserted into the food contained in the vessel 41 and theplug 43 is insertedinto the receptacle 38 carried by the backsplash i3.The'handle 16 of the circuit controller 31 is then set into the desiredkffat position or candy position `corresponding to the cooking operationthat is to be carried outin the vesself41. Assuming that'a candycookingoperation is to be'carried out in the vessel 41 on the hot plate 24, thehandle 15 is set into its candy position and the hot plate switch 29 isoperated to effect heating of the hot plate 24 that supports the vessel41. When the circuit controller 31 is set into its candy position, theblade94 engages' the candy contact in the associ-ated contact bankshort-circuiting the resistorl |05 and completing a direct connectionbetween the conductors 99 and 98, whereby the normal negative biasapplied between the control grid and the cathode 83 of the tube 80 isappropriately and correspondingly increased to effect matching of thetemperature sensing resistor 52 and the candy scale 12 so that it is atthis time framed in the window |52.l The operation lof the circuitcontrol network is the same as that previously described, exceptthatwith the increased negativeY bias normally applied between thecontrol grid A85 and the cathode 83 of the tube 80, theplate-cathodecurrent of the tube' is reduced at a lower temperaturerange oftheftemperature sensing resistor 52 so as to match lthetemperaturesensing resistor 52 to the candy'scale 12, wherebythe `meter36 `reads substantially midscale-whenv the temperature of thetemperature sensing resistor 52 is at the midpoint temperature of 270 F.in the temperature range 220 to 320 F.,

the midpoint temperature noted corresponding to the -temperatureat whicha hard ball candy cookingeoperation is normally carried out. Of courseit will be understood that in carrying out a candy cooking operation'thecandy syrup being heated is maintained at the required tempera* tureduring an appropriateftime interval in accordance with conventionalpractice.-

In viewof the above, it will be understood that a fat frying cookingoperation may be carried out in the vessel v41 in a substantiallyidentical manner while the probe 42 is immersed in the fat and the plug43 is inserted into the receptacle 38. However, in this case the handle16 of the circuit controller 31 is rotated into its fat position inorder to bring about framing 4of the fat scale 1I in the window 62 andthe engagement'by the blade 94 of thev fat contact in its associatedcontact bank. At this time the blade 94 engaging the fat contact in theassociated contactbank short-circuits both of the resistors |04 and |05completinga directconnection be 1l 370 F., lying substantially in themidpoint of the range BZW-420 F.

In view of the foregoing description, it will be understood that aselecting movement of the manually operable handle 'l5 of the circuitcontroller 3l not only selects the required scale 1l, etc., of the meter35 framing it in the window 62, but it also simultaneously sets theblade 84 in order to set the appropriate negative bias between thecontrol grid 85 and the cathode 83 of the tube 80, whereby thetemperature sensing resistor 52 is matched against the selected Scale1I, etc., in order that the meter 35 may read across the entire selectedscale 1|, etc., and so that the mid-temperature point of the selectedscale 1i, etc., corresponds to the mid-temperature range of thetemperature sensing resistor 52 in carrying out the particularcorresponding cooking operation.

Reconsidering the circuit network of Fig. 2, it is pointed out thatwhile the tubes 8D and 8l oi the respective types l2AV6 and 50C5 areconstructed and arranged to have substantially dat plate current outputswith small variations in plate voltages within the normal operatingranges thereof, there is nevertheless a small increase in plate currentin response to a reasonable increase in plate voltage from normal ineach of these tubes, whereby a variation from normal voltage of theEdison source induces a small variation in the response of the meter 35to measure the temperature of the temperature sensing resistor 52. Inorder to minimize this effect and further to hold substantially flat theplate current characteristics of the tubes 89 and 8l even though thevoltage of the Edison source varies somewhat from normal, the ballastresistor |33 is arranged in the series heating circuit including theheaters 84 and 85 operatively associated with the respective cathodes 83and 8l of the respective tubes 35i and Bl. This arrangement employingthe ballast resistor |03 having the high positive coeicient ofresistance prevents any substantial increase in the current traversingthe heaters 84 and 88 notwithstanding a reasonable increase in thevoltage of the Edison source above normal, whereby the heater currentremains substantially constant preventing increased electron emissionsfrom the oxide coated cathodes 83 and 81 of the respective tubes 8i! and8l in response to increases in plate voltages. Accordingly it will beunderstood that this arrangement compensates for the small error thatwould be otherwise induced in the reading of the meter 35 when thevoltage of the Edison source varies somewhat above normal.

In view of the foregoing, it is apparent that there has been provided inan electric range, an improved electrical temperature indicating systemthat is simple and eiiicient in operation permitting the cook to carryout a wide range of cooking operations both in the oven of the range andon a hot plate thereof, in an ei'licient `and simple manner.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. An electric temperature indicating system comprising a single phasealternating current source of the three-wire Edison type including twooutside conductors and a neutral conductor, a first resistor, a rstelectron discharge tube including an anode and a cathode and a grid, aconnection via said rst resistor between said neutral conductor and theanode of said rst tube, a connection between one of said outsideconductors and the grid of said rst tube, a sec` ond resistor having asubstantial positive temperature coefficient of resistance, a connectionvia said second resistor between said one outside conductor and thecathode of said first tube establishing a negative bias between the gridand the cathode of said first tube so that said negative bias isincreased as the resistance of said second resistor is increased withthe temperature thereof in order to reduce the current traversing saidrst resistor, a second electron discharge tube including an anode and acathode and a grid, a connection between the other of said outsideconductors and the anode of said second tube, a connection between saidneutral conductor and the cathode of said second tube, a meter includedin the anode-cathode circuit of said second tube and calibrated in termsof the temperature of said second resistor, and a connection via saidfirst resistor between said neutral conductor and the grid of saidsecond tube establishing a negative bias between the grid and thecathode of said second tube so that said last-mentioned negative bias isreduced as the current traversing said rst resistor is reduced in orderto increase the current traversing said meter.

2. An electric temperature indicating system comprising a single phasealternating current source of the three-wire Edison type including twooutside conductors and a neutral conductor, a first resistor, a rstelectron discharge tube including an anode and a cathode and a grid, aconnection via said first resistor between said neutral conductor andthe anode of said iirst tube, a connection between said one outsideconductor and the grid of said first tube, a Second resistor having asubstantial positive temperature coeicient of resistance, a thirdresistor, a connection via said second and third resistors in seriesrelation between said one outside conductor and said neutral conductor,a connection between the junction of said second and third resistors andthe cathode of said first tube establishing a negative bias between thegrid and the cathode of said first tube so that said negative bias isincreased as the resistance of said second resistor is increased withthe temperature thereof in order to reduce the current traversing saidrst resistor, a second electron discharge tube including `an anode and acathode and a grid, a connection between the other of said outsideconductors and the anode of said second tube, a connection between saidneutral conductor and the cathode of said second tube, a meter includedin the anode-cathode circuit of said second tube and calibrated in termsof the temperature of said second resistor, and a connection via saidrst resistor between said neutral conductor and the grid of said secondtube establishing a negative bias between the grid and the cathode ofsaid second tube so that said last-mentioned negative bias is reduced asthe current traversing said rst resistor is reduced in order to increasethe current traversing said meter.

3. An electric temperature indicating system comprising a single phasealternating current source of the three-wire Edison type including twooutside conductors and a neutral conductor,

a first resistor, a first electron discharge tube including an anode anda cathode and a grid, a connection via said first resistor between saidneutral conductor and the anode of said rst tube, a connection betweensaid one outside conductor and the grid of said first tube, a secondresistor having a substantial positive temperature coefficient ofresistance, a third resistor, a connection via said second and thirdresistors in series relation between said one outside conductor and thecathode of said rst tube establishing a negative bias between the gridand the cathode of said rst tube so that said negative bias is increasedas the resistance of said second resistor is increased with thetemperature thereof in order to reduce the current traversing said firstresistor, means for selectively varying the resistance of said thirdresistor in order selectively to Vary said negatiVe bias andconsequently the current traversing said rst resistor, a second electrondischarge tube including an anode and a cathode and a grid, a connectionbetween the other of said outside conductors and the anode of saidsecond tube, a connection between said neutral conductor and the cathodeof said second tube, a meter included in the anode-cathode circuit ofsaid second tube and calibrated in terms of the temperature of saidsecond resistor, and a connection via said first resistor between saidneutral conductor and the grid of said second tube establishing anegative bias between the grid and the cathode of said second tube sothat said last-mentioned negative bias is reduced as the currenttraversing said first resistor is reduced in order to increase thecurrent traversing said meter.

4. The electric temperature indicating system set forth in claim l, andfurther comprising means for preventing the currents in theanode-cathode circuits of said rst and second tubes from increasingproportionally with an increase in voltage above normal between saidoutside conductors.

5. The electric temperature indicating system set forth in claim l,wherein the cathode of each of said first and second tubes is of theoxidecoated type and is provided with an associated cathode heater, andfurther comprises a cathode heater circuit including in series relationthe cathode heaters of said first and second tubes and a ballastresistor having a high positive temperature coeicient of resistance.

6. The electric temperature indicating system set forth in claim l, andfurther comprising a plug-connector carried by said second resistor anda plurality of socket-connectors included in said connection betweensaid one outside conductor and the cathode of said first tube, each ofsaid socket-connectors accommodating ready placement and removal of saidplug-connector with respect thereto.

7. The electric temperature indicating system set forth in claim 2,wherein said meter is provided with rst and second scales respectivelycalibrated in terms of first and second ranges of the temperature ofsaid second resistor, and further comprising a switch having rst andsecond settings respectively establishing first and second resistancevalues of said third resistor in order to set corresponding first andsecond negative biases so as to match said second resistor to said firstand second scales, and manually operable means for selectively operatingsaid switch into either one of its positions and for simultaneouslyselecting the corresponding one of said scales.

GEORG-El W. SCHROEDER.

References Cited in the le of this patent UNITED STATES PATENTS FranceOct. 28, 1920

